ISL6545 Datasheet, PDF (14/16 Pages) Intersil Corporation – 5V or 12V Single Synchronous Buck Pulse-Width Modulation (PWM) Controller

ISL6545, ISL6545A
+V_CC
+V_IN
VCC
+ V_D -
ISL6545
BOOT
C_BOOT
Q1
UGATE
PHASE
VCC
Q2
-
+
LGATE/OCSET
GND
V_G-S ≈ V_CC - V_D
V_G-S ≈ V_CC
FIGURE 11. UPPER GATE DRIVE BOOTSTRAP
BOOTSTRAP Considerations
Figure 11 shows the upper gate drive (BOOT pin) supplied by
a bootstrap circuit from V_CC. The boot capacitor, C_BOOT,
develops a floating supply voltage referenced to the PHASE
pin. The supply is refreshed to a voltage of V_CC less the boot
diode drop (V_D) each time the lower MOSFET, Q₂, turns on.
Check that the voltage rating of the capacitor is above the
maximum V_CC voltage in the system; a 16V rating should be
sufficient for a 12V system. A value of 0.1µF is typical for
many systems driving single MOSFETs.
If V_CC is 12V, but V_IN is lower (such as 5V), then another
option is to connect the BOOT pin to 12V, and remove the
BOOT cap (although, you may want to add a local cap from
BOOT to GND). This will make the UGATE V_GS voltage
equal to (12V - 5V = 7V). That should be high enough to
drive most MOSFETs, and low enough to improve the
efficiency slightly. Do NOT leave the BOOT pin open, and try
to get the same effect by driving BOOT through V_CC and the
internal diode; this path is not designed for the high current
pulses that will result.
For low V_CC voltage applications where efficiency is very
important, an external BOOT diode (in parallel with the
internal one) may be considered. The external diode drop
has to be lower than the internal one; the resulting higher
V_G-S of the upper FET will lower its r_DS(ON). The modest
gain in efficiency should be balanced against the extra cost
and area of the external diode.
14
FN6305.5
April 29, 2010